In a fabrication process for forming a 64 Mbit DRAM device, a sidewall nitride etching step produces a round shoulder in a silicon nitride, Si.sub.3 N.sub.4, dielectric material layer covering a conductive stripe, such as a wordline. The dielectric material is placed there to provide sufficient electrical insulation to maintain separation between the wordline and other conductive material, such as in a bitline contact or a storage node contact. Because of the round shoulder in the silicon nitride, the shortest distance from a corner of the wordline to the bitline contact or the storage node contact is at a minimum to maintain the desired electrical separation of the two conductors.
However, the fabrication process proceeds with, for example, a storage node contact etch step. Because of the shape of the round shoulder in the silicon nitride, oxidenitride selectively is reduced at that shoulder during the storage node contact etch. Consequently, the thickness of the silicon nitride is reduced below the desired minimum and an undesirable short circuit may occur between the wordline and the bitline contact or the storage node contact.
Current literature describes several chemistries for etching silicon nitride. Characteristics and properties of etching silicon nitride with either carbon fluoride, CF.sub.4, sulfur fluoride, SF.sub.6, or nitrous fluoride in an inert mixture are described. Information presented includes the etch rate and selectivity. No information describes the effect of etching on the profile of the sidewall silicon nitride. That profile, however, is very important for developing a useful silicon nitride insulating structure between two conductors where at least one of the conductors has a sidewall and top surface covered with the silicon nitride.